In this work, normal metal and superconducting tunnel junctions have been studied. In particular applications in low temperature electronic thermometry are investigated. The double oxidation method for achieving high resistance junctions is described and shown to produce high quality devices. A new invention, the single junction thermometer (SJT), is described, and proof of the concept experiments demonstrating the functionality of the device are described. A key feature of the SJT is a well controlled electromagnetic environment surrounding a single junction. It is shown that embedding a single junction within arrays of other junctions is a superior way for controlling the environment as compared to previously proposed methods. Improved thermalisation of electrons is shown to be beneficial for the performance of superconducting RSFQ (Rapid Single Flux Quantum) devices. The so called grey zone of the most basic of these devices, the balanced comparator, is investigated. Improved performance based on metallic cooling fins has been achieved. In the same spirit improved thermalisation of the Coulomb blockade thermometers extends their operation towards lower temperatures. A method employing thick metallic islands was developed and tested to this end.
In this work, normal metal and superconducting tunnel junctions have been studied. In particular applications in low temperature electronic thermometry are investigated. The double oxidation method for achieving high resistance junctions is described and shown to produce high quality devices. A new invention, the single junction thermometer (SJT), is described, and proof of the concept experiments demonstrating the functionality of the device are described. A key feature of the SJT is a well controlled electromagnetic environment surrounding a single junction. It is shown that embedding a single junction within arrays of other junctions is a superior way for controlling the environment as compared to previously proposed methods. Improved thermalisation of electrons is shown to be beneficial for the performance of superconducting RSFQ (Rapid Single Flux Quantum) devices. The so called grey zone of the most basic of these devices, the balanced comparator, is investigated. Improved performance based on metallic cooling fins has been achieved. In the same spirit improved thermalisation of the Coulomb blockade thermometers extends their operation towards lower temperatures. A method employing thick metallic islands was developed and tested to this end.
This Thesis studies opportunities to create a quantum standard for the electric current with the help of tunnel junctions. We use two types of tunnel junctions: superconducting Josephson junctions, and NIS junctions where one of the electrodes is normal (N) and the other one superconducting (S). In both cases, tunnel junctions are employed in a single-electron transistor (SET) structure, which is used to transfer a controlled number k of electrons (e) with the repetition frequency f. The magnitude of the resulting current is thus I = kef. First, we study the Cooper pair sluice, where Josephson junctions are connected as two superconducting quantum interference devices (SQUID). A gate electrode is used to control the charge state of the superconducting island formed between the SQUIDs. The sluice is based on tuning the tunneling rates through the SQUIDs with local magnetic fluxes, which allows to control the direction of the charge transfer. Weak traces of current quantization can be observed up to above 1 nA, which is large enough current for many metrological purposes. However, the accuracy of the current is still far from what is required in metrology. We study also a new type of SQUID structure, the balanced SQUID, which could be used to improve the accuracy of the sluice or in, e.g., some quantum computing applications. Second, we employ NIS junctions in the hybrid (SINIS) single-electron transistor with superconducting leads and a normal-metal island. This structure can be used as the SINIS turnstile which lets electrons to flow one by one in the direction determined by the bias voltage. We report the first experimental results on the SINIS turnstile and two methods to improve the accuracy of the current: increasing the charging energy of the island or connecting the turnstile in a resistive environment. We also show that the SINIS turnstile can be used as an electronic radio-frequency refrigerator where tunneling processes cool the electron temperature of the normal-metal island.
In this work four different mesoscopic superconducting devices have been experimentally studied: an ammeter based on a hysteretic Josephson junction switching from the superconducting state to the normal state, a conventional Cooper pair pump (CPP) based on two superconducting islands separated by tunnel junctions, a novel flux assisted Cooper pair pump and a thermometer based on a tunnel junction between a superconductor and a normal metal. These devices make use of phenomena related to superconductivity and are also benefiting from some properties of small structures, e.g., of quantum mechanical tunneling and of transport of single electrons. The present research focused on investigating performance limitations of these devices and the aim was to improve their performance. A hysteretic Josephson junction switching can be used as a sensitive current threshold detector. For many purposes, a smaller critical current Ic of the detecting junction would yield a higher current sensitivity. It was found in this Thesis that the main limitation of the ammeter is that with increasing sensitivity the negative effects due to dissipation start to dominate. In this Thesis the Josephson junction was also demonstrated as a shot-noise detector, and it was concluded that this kind of a device could probably be made into an absolute on-chip detector of Fano-factors and noise in general. A charge pump is a device which can periodically transfer a discrete amount of charge Q through an electrical circuit. The measured double island CPP introduced a frequency dependent current with a value close to the expected one at low pumping frequencies. The drawback in the measurements was that the direction of the pumped current was set by the bias voltage. The results of the measurement on flux assisted pumping were indeed very promising and the studied device was able to produce sufficiently high pumped currents of around 0.1 nA with reasonable accuracy. Thermometer based on NIS (Normal metal - Insulator - Superconductor) tunnel junctions is a very promising temperature sensor for bolometric radiation sensor applications. This work showed that it can be used as a sensitive thermometer up to MHz frequencies range and that its response time is limited by the electron-phonon scattering rate.
The performance and accuracy of quantum electronics is substantially degraded when the temperature of the electrons in the devices is too high. The electron temperature can be reduced with appropriate thermal anchoring and by filtering both the low frequency and radio frequency noise. Ultimately, for high performance filters the electron temperature can approach the phonon temperature (as measured by resistive thermometers) in a dilution refrigerator. In this application note, the method for measuring the electron temperature in a typical quantum electronics device using Coulomb blockade thermometry is described. This technique is applied to find the readily achievable electron temperature in the device when using the QFilter provided by QDevil. With our thermometry measurements, using a single GaAs/AlGaAs quantum dot in an optimized experimental setup, we determined an electron temperature of 28 ± 2 mK for a dilution refrigerator base temperature of 18 mK.
This thesis presents results focused on fabrication and electrical characterization of superconducting tunnel junctions and Josephson junctions, e.g. measuring the current–voltage response and 1/f noise from different samples at different temperatures. Junctions were fabricated inside a vacuum chamber where metal evaporation through a resist mask was used to create submicron-sized junctions. The tunneling barrier was common between all different samples, i.e. in-situ oxidized aluminium between the metal depositions. Different material combinations were used, e.g. Al–AlOₓ–Al, Al–AlOₓ–Al:Mn and Al–AlOₓ–Nb. The manganese doping in aluminium (Al:Mn) suppresses the superconductivity making the Al:Mn parts normal, while pure aluminium is superconducting, creating a normal metal–insulator–superconductor (NIS) device. The qualities of junctions were improved, if possible, by vacuum thermal annealing at 400 °C, which was found both to stabilize aluminium and aluminium–manganese based junctions and to reduce the characteristic 1/f noise in them. These results are desired, since for most applications it is important to have a junction with stable parameters over long time and 1/f noise is limiting the sensitivity of many devices. However, the procedure is not for granted, since most other NIS material combinations didn't survive the annealing treatment. We studied Al–AlOₓ–Nb junctions at cryogenic temperatures (≈ 100 mK) either as an SIS' device, or as an NIS device by suppressing the superconductivity of aluminium with magnetic field, and observed excess sub-gap current, which could not be explained by standard tunneling theory with the Dynes model.
We experimentally demonstrate the high bandwidth readout of a thermometer based on a superconductor–insulator–normal metal (SIN) tunnel junction, embedded in a rf resonant circuit. Our implementation enables basic studies of the thermodynamics of mesoscopic nanostructures. It can also be applied to the development of fast calorimeters, as well as ultrasensitive bolometers for the detection of far-infrared radiation. We discuss the operational details of this device, and estimate the ultimate temperature sensitivity and measurement bandwidth.
The work involves a study of physical phenomena that take place at very small length scales, below one micro-meter. At temperatures roughly below one degree Kelvin, quantum-mechanical effects may rule in electronic transport. Macroscopic quantum-coherent effects that occur in metallic superconducting microstructures, are particularly intriguing. Large-scale quantum information processing is widely believed to be attainable utilizing such physical systems. This work concentrates on answering the question of how the described quantum-mechanical systems may be used as sensitive measuring devices. Considerable attention is paid to energy-storing metallic microstructures whose electrical properties resemble those of the familiar inductor or capacitor. This research involves primarily experimental investigations conducted around temperatures of 0.1 Kelvin. Methods both at low and at radio frequencies have been used. The experimental findings have been modelled theoretically, and theoretical concepts for new physical phenomena have been introduced. An inductively measured radio-frequency Cooper-pair transistor, the L-SET, has been developed and experimentally verified in this work. Being highly sensitive, fast, and non-invasive, the L-SET appears to be the most promising method for measuring electric charge. Sensitivity in charge measurements of 20 millionths of the electron charge (micro-e) within one second, and an input bandwidth of 100 MHz, have been demonstrated. It has been shown theoretically that the ultimate measurement accuracy is about 0.1 micro-e within a second. A new phase detector based on the Cooper-pair transistor has been proposed. This system has also been shown to be potentially usable as a quantum bit. A new type of radio-frequency single-electron transistor built using a multi-walled carbon nanotube has been fabricated and operated. Technologies have been developed in order to make the physical nano- or microstructures. A method has been presented to fabricate non-superconducting tunnel junctions. Consequences of the inverse superconducting proximity effect on the studied superconducting structures were considered. Measurement procedures were investigated for a new low-noise nanoamplifier, the Bloch-oscillating transistor. Single superconducting tunnel junctions were tested as detectors of energy states of the environment, or of noise.